9. An overall appraisal of the health and psychological effects of cannabis

9.1 Summary

The following is a summary of the major adverse health and psychological effects of acute and chronic cannabis use, grouped according to the degree of confidence in the view that the relationship between cannabis use and the adverse effect is a causal one.

9.1.1 Acute effects

The major acute psychological and health effects of cannabis intoxication are:

anxiety, dysphoria, panic and paranoia, especially in naive users;

cognitive impairment, especially of attention and memory for the duration of intoxication;

psychomotor impairment, and probably an increased risk of accidental injury or death if an intoxicated person attempts to drive a motor vehicle or operate machinery;

an increased risk of experiencing psychotic symptoms among those who are vulnerable because of a personal or family history of psychosis; and

an increased risk of low birth weight babies if cannabis is used during pregnancy.

9.1.2 Chronic effects

The major health and psychological effects of chronic cannabis use, especially daily use over many years, remain uncertain. On the available evidence, the major probable adverse effects appear to be:

respiratory diseases associated with smoking as the method of administration, such as chronic bronchitis, and the occurrence of histopathological changes that are precursors to the development of malignancy;

development of a cannabis dependence syndrome, characterised by an inability to abstain from or to control cannabis use; and

subtle forms of cognitive impairment, most particularly of attention and memory, which persist while the user remains chronically intoxicated, and may or may not be reversible after prolonged abstinence from cannabis.

The following are the major possible adverse effects of chronic, heavy cannabis use which remain to be confirmed by controlled research:

an increased risk of developing cancers of the aerodigestive tract, i.e. oral cavity, pharynx, and oesophagus;

an increased risk of leukemia among offspring exposed in utero; and

a decline in occupational performance marked by underachievement in adults in occupations requiring high level cognitive skills, and impaired educational attainment in adolescents.

birth defects occurring among children of women who used cannabis during their pregnancies.

9.1.3 High risk groups

A number of groups can be identified as being at increased risk of experiencing some of these adverse effects.

Adolescents

Adolescents with a history of poor school performance may have their educational achievement further limited by the cognitive impairments produced by chronic intoxication with cannabis.

Adolescents who initiate cannabis use in the early teens are at higher risk of progressing to heavy cannabis use and other illicit drug use, and to the development of dependence on cannabis.

Women of childbearing age

Pregnant women who continue to smoke cannabis are probably at increased risk of giving birth to low birth weight babies, and perhaps of shortening their period of gestation.

Women of childbearing age who continue to smoke cannabis at the time of conception or while pregnant possibly increase the risk of their children being born with birth defects.

Persons with pre-existing diseases

Persons with a number of pre-existing diseases who smoke cannabis are probably at an increased risk of precipitating or exacerbating symptoms of their diseases. These include:

individuals with cardiovascular diseases, such as coronary artery disease, cerebrovascular disease and hypertension;

individuals with respiratory diseases, such as asthma, bronchitis, and emphysema;

individuals with schizophrenia, who are at increased risk of precipitating or of exacerbating schizophrenic symptoms; and individuals who are or have been dependent upon alcohol and other drugs, who are probably at an increased risk of developing dependence on cannabis.

9.1.4 A caveat

As has been stressed throughout this document, there is uncertainty surrounding many of these summary statements about the adverse health effects of acute, and especially chronic, cannabis use. To varying degrees, these statements depend upon inferences from animal research, laboratory studies, and clinical observations about the probable ill effects. In some cases, the inferences depend upon arguments from what is known about the adverse health effects of other drugs, such as tobacco and alcohol. In very few cases are there sufficient studies which provide the detailed evidence that epidemiologists would require to make informed judgments about the health effects of cannabis; the interpretation of what epidemiological evidence is available is complicated by difficulties in quantifying degree of exposure to cannabis, and in excluding alternative explanations (including other drug use) of associations observed between cannabis use and adverse health outcomes. These interpretative problems are especially obvious in the case of many of the alleged psychological outcomes of cannabis use in adolescence, since many of these putative "consequences" (e.g. poor school performance, deviant behaviour) also antedate the use of cannabis. Nevertheless, these statements provide the best available basis for making societal decisions about what policies ought to be adopted towards cannabis use.